1. Field of the Invention
The present invention relates to an improved process and apparatus for blow molding. In particular, it relates to a process and apparatus producing molded hollow articles formed of an engineering plastic material.
2. Description of the Related Art
The blow molding process is a well known method of producing hollow plastic articles, and according to a prior art, an extrusion machine having a die defining a downwardly extending annular melt passage is used in combination with air blowing means, a mold provided below the die including two mold halves defining a cavity, an article removing device as shown in FIGS. 1 and 3 for pinching a parison forming a molded article at a portion thereof above the mold, and removing the parison when the mold is opened, and a pinching device for pinching the parison at a free end thereof.
The process comprises the steps of: plasticizing and metering a plastic material within the machine, extruding the plasticized and metered material in a hollow form through the die annular passage toward the mold, when the mold is opened, preblowing air at a lower pressure from the die into the interior of the extruded hollow material during the extruding step, to produce a parison having a longitudinal hollow form, to be partially received in the mold and molded to form the article within the cavity. The mold is cooled while the mold halves are clamped to each other with the parison therebetween, and air is blow into the parison at a higher pressure from the mold, following the preblowing. When the mold is opened the parison is pinched or grasped by the removing device, and after the mold is opened, the parison is removed by the device, with the result that the parison is drawn from or torn off the die at a free end thereof.
A next extrusion is commenced at a lower extrusion rate or speed, after the instant parison is separated from the die, to produce a next or succeeding parison. A leading portion of the next parison formed due to the initial stage of the next extruding step at the lower speed is self-adhered by the pinching device at its free end, whereby the free end is closed for a next preblowing step with air into the next parison to be completed. After the pinching operation, the extrusion is switched from the lower speed to a higher speed to produce a following portion of the next parison, and the next preblowing step is commenced concurrently with the later stage of the next extruding step at the higher speed, to complete the next parison.
The above conventional blow molding process is illustrated in FIGS. 5(a) and (c) and FIG. 8, wherein (X+Y) corresponds to a complete parison consisting of both leading and following portions, .DELTA.T is the period of time of the initial extrusion stage at the lower speed .beta., and .alpha. denotes the higher extrusion speed. The leading and following portions of the parison in combination form a downstream portion (X) and an upstream portion Y. The upstream portion (Y) extends from the free end of the die to a pinching position above and in the vicinity of the mold. The downstream portion x is partially received in the mold with at least the closed and free lower end of the parison, i.e., a lower end portion (.DELTA.X.sub.2) of the downstream portion (X) being excluded from the cavity at a lower end thereof, and an upper end portion (.DELTA.X.sub.1) of the downstream portion (X) being excluded from the cavity at an upper end thereof and extending upwardly from the top of the mold.
The upper end portion (.DELTA.X.sub.1) and a lower end portion (.DELTA.X.sub.2) of the downstream parison portion (X), and the upstream parison portion (Y) integrated with said upper end portion (.DELTA.X.sub.1) are all wasted, after the molded article is separated therefrom by the removing device, provided that said lower end portion (.DELTA.X.sub.2) is excluded from the cavity. The initial extrusion stage at the lower speed .beta. until termination of the pinching operation is carried out over a time of ##EQU1## and the later extrusion stage at the higher speed .alpha. and the preblowing step are concurrently carried out over a time of ##EQU2## The downstream portion X of the parison is prepared in both the initial and later extrusion stages over a time of ##EQU3## and the upstream portion Y of the parison is prepared in the later extrusion stage at the higher speed .alpha. over a time of ##EQU4## wherein: ##EQU5##
Recently, engineering plastic materials have been used as advantageous materials for plastic blow molded articles, as they have a high thermal resistance. But a physical melt property thereof causes a hot parison to be drawn down or elongated excessively relative to that of normal plastic materials. In this regard, a high extrusion, i.e., an extrusion at a high rate or speed, is desired to prepare a better parison to be received in a shorter time in the mold, for producing an article with a higher quality, but as stated above, the extrusion must be carried out at the lower speed .beta. rather than the higher speed .alpha. to allow the pinching device to self-adhere by a free end of a new parison.
In this regard, the initial extrusion stage at the lower speed .beta. is not preferable for producing a desired parison, and it is desired that such a parison be prepared at a higher average extrusion speed from the beginning to thereby improve the quality of articles and shorten a shot cycle or time of a blow molding run, to thus increase the productivity of the blow molding, if possible.
The above mentioned upstream portion (Y) of the parison (X+Y) must ensure that the downstream portion (X) is partially received in the mold such that at least the free lower end or the lower end portion (.DELTA.X.sub.2) is excluded from the cavity at the lower end thereof, and at least the upper end portion (.DELTA.X.sub.1) is excluded from the cavity with the upper end thereof extended form the top of the mold, and with the main part properly received in the cavity. A length of the upstream portion (Y) relies on a diameter of a parison of a circle form or shorter and lower diameters of a parison of an elliptic form. In general, the longer the diameter of the circular parison, the longer the upstream portion must be for a desired blow molding. The upstream portion (Y) is also required to provide a space gap between the die and the mold sufficient to allow the drawing and removal operation to be carried out, as stated above.
For example, with a circular parison having a diameter of 350 mm or an elliptical parison having a shorter diameter of 350 mm, the upstream portion Y has at least a length equivalent to the parison diameter (350 mm), and a length equivalent to a parison part (.DELTA.Z) between the mold and the upper parison portion Y, as shown in FIG. 5(b) is 100 to 150 mm.